The DRM schemes rely on time in order to verify the rendering rights. This results in the necessity of securing the clock in order to force correct application of the content protection rules. For a mobile equipment the securing of the clock is rendered difficult by various issues, e.g. the mobile equipment might not be necessarily registered to a network, the user needs to change the clock in order to adapt it to a different time zone or the user has to set a new time value in case the real time is lost for example during a change of battery. A time control system for mobile equipments is therefore required, which allows the user to change the clock under certain circumstances but limits the possible fraud made to a level, which is considered acceptable, without limiting the functionality of the mobile equipment.
A time recording and controlling system for a camera is revealed in document U.S. Pat. No. 5,526,079. It discloses a data recording system for a camera, which records information such as the time, the data, the name and a corporate symbol onto a recording medium used by the camera. If these data are lost, new values must be set by the user. A control system determines, if the new data are considered correct, in this case the new data are accepted. Otherwise the new time and date are not used and the data recording system is stopped or the camera is blocked and cannot be used.
The disadvantage of this system is the possible limited functionality of the camera after changing the data, e.g. when the camera still works but without the date and time function or when the camera is shut off completely.